This application is based on and incorporates herein by reference Japanese Patent Application No. 2000-149893 filed on May 22, 2000.
1. Field of the Invention
The present invention relates to a radio communication system capable of data transmission between a mobile terminal station installed on a mobile object (e.g., a vehicle) and fixed base stations fixedly provided along a traveling route (e.g., a roadside) of the mobile object.
2. Related Art
As a radio communication technique in the field of mobile communication, a cellular communication system such as a PDC (Personal Digital Cellular) system have already been realized. In PDC system, the communication service area is divided into a number of macro cells, and a fixed radio base station is installed in the center of each cell. To realize a continuous type communication, each cell must be overlapped with adjacent cells. Thus the carrier frequency of each cell must be different from those of the adjacent cells to prevent interference. Accordingly the whole frequency range allocated to the service (PDC system) should be divided and allocated to each cell. In this case, the frequency band to be used by each user becomes narrow in comparison with the whole band, and as a result the possible rate of data transmission is limited. Further, a plurality of user mobile terminals exist in each cell, if the cell is defined relatively large. Accordingly multiple access should be allowed in each cell. For example, time division multiple access (TDMA), frequency division multiple access (FDMA), or code division multiple access (CDMA) is adopted for allocating the time for access to each user, and thereby the average rate of data transmission is further lowered.
Thus, when a cellular communication system is used for communication between a vehicle and a roadside, the communication resources (time, frequency or code) allocated to each user is insufficient for broadband communication. Therefore it is difficult to implement broadband communication by the cellular communication system. Japanese patent application No. 2000-72387 (U.S. patent application Ser. No. 09/804,475 filed on Mar. 13, 2001) proposes a novel communication system which is effective for implementing broadband communication. It also proposes a technique for providing an improved data transmission rate in comparison with cellular communication system and allowing to reduce the number of base stations. In this technique, data is divided and transmitted in a plurality of communication areas formed by a plurality of base stations installed along a traveling route of a vehicle if data transmission cannot be completed in one communication area formed by one base station.
In the case that data is thus divided and transmitted in a plurality of communication areas, traveling speed of a vehicle on which a terminal station is installed should be considered for efficient data transmission. The vehicle traveling speed or the time of the vehicle stay in a communication area greatly depends on a degree of traffic congestion. Therefore, if the length of a packet is fixed based on the time of the vehicle stay in a communication area in case of traffic congestion of a specific degree, it is too long or too short when the actual degree of traffic congestion is different from the specific degree. That is, transmission of a packet is completed too soon and time is left uselessly (the fixed length of a packet is too short) if the actual time of the vehicle stay in a communication area is longer than the assumed time, but transmission of a packet cannot be completed (the fixed length of a packet is too long) if the actual time of the vehicle stay in a communication area is shorter than the assumed time. Further, the time of the vehicle stay in a communication area also depends on a traveling speed of the vehicle. If the length of a packet is fixed, it should be fixed based on the time of the vehicle stay in a communication area in the case that the vehicle travels at the possible highest speed. However, it is too short in the case that the vehicle travels at a lower speed than the possible highest speed.
The above problem arises not only in the radio communication system wherein a spot access method, which forms communication areas not to overlap each other, is employed similarly as proposed. It may also arise in the communication system wherein communication areas are formed to overlap each other, if data transmission involving data division is performed in the communication system. Further, the above problem arises not only in the case that data is transmitted from base stations to a terminal station. It may also arise in the case that data is transmitted from a terminal station to base stations, if the data transmission involves data division.
Therefore, the length of a packet should be adjusted according to the time of the vehicle stay in the communication area. For that purpose, the traveling speed of the vehicle needs to be detected. This may be obtained from information provided by a vehicle speed sensor or by using GPS. More accurate detection method, if any, is more desirable in the case that the communication areas are relatively narrow (that is, the time of the vehicle stay in the communication area is relatively short). Furthermore, it is desirable to detect the vehicle traveling speed by utilizing the radio communication system which provides communication between a terminal station and base stations, since it is not required to prepare a special structure for detecting the traveling speed.
The present invention has an object to provide a radio communication system which performs radio communication between a mobile terminal station and fixed base stations using a packet of an appropriate length according to the time of the terminal station stay in each communication area formed by each base station.
The radio communication system according to the present invention performs radio communication between a terminal station installed on a mobile object and a plurality of base stations provided along a traveling route of the mobile object. If the data transmission between the base station and the terminal station cannot be completed in one communication area formed by the base station, data is divided and transmitted in a plurality of communication areas corresponding to base stations arranged in a traveling direction of the mobile object. When the data transmission involving data division is performed in this way, the present system detects the traveling speed of the mobile object in each communication area and estimates the time of the mobile object stay in the communication area based on the detected traveling speed of the mobile object.
The length of a packet is determined based on the estimated time of the mobile object stay in the communication area so that transmission of the packet can be completed during the mobile object stay in the communication area and is not completed too soon. The length of the packet is adjusted to the determined length by varying the number of frames included in the packet or varying the length of a frame. In this way, the present system performs data transmission using a packet of an appropriate length according to the detected mobile object traveling speed.
The present invention also provides several methods for detecting the traveling speed of a mobile object accurately by utilizing the present radio communication system.